1. Field of the Invention
The present invention relates to a method for controlling a vertical type Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET) in a bridge circuit.
2. Description of Related Art
A reverse recovery current due to switching of a MOSFET is one factor in power loss and restriction of high speed switching. According to a conventional configuration to reduce the reverse recovery current, a fast recovery diode (FRD) or a Schottky barrier diode (SBD) is connected in parallel with the MOSFET, and a reverse blocking diode for disabling a body diode is connected with the MOSFET. The above configuration may reduce the switching loss and increase a switching speed. The parallel-connected external SBD is used for suppressing voltage drop across a built-in diode of the MOSFET approximately from 2.0V to 0.4 V.
Connection of the reverse blocking diode according to the conventional manner, however, requires an additional reverse blocking diode. An on-state resistance of the MOSFET may increase, and a circuit may be complicated and enlarged.
A technique for suppressing a reverse recovery phenomenon without using a reverse blocking diode has been proposed in Japanese Unexamined Patent Application Publication Number H7-194105. According to the technique, a lateral type MOSFET includes an insulated gate provided separately from a back gate, and the back gate is reverse-biased.
The technique disclosed in Japanese Unexamined Patent Application Publication Number H7-194105 is applicable to a lateral type MOSFET but may be unsuitable for a vertical type MOSFET, which is frequently used for high power application. Moreover, an additional control terminal and an additional reverse bias power supply are required, which may lead to complication and enlargement of a circuit.
Furthermore, a self turn-on phenomenon may occur easier in a case of a MOSFET having a large ratio of feedback capacity to input capacity. In the self turn-on phenomenon, a gate voltage of a MOSFET instantaneously exceeds a threshold voltage in case of switching the MOSFET.